(or Rockwell Collins) 651-S1 was one of the first of what could be termed "the modern design". The unit was developed for goverment/military application. The design included a interface to allow computer control. The early unit used the gas discharge tube digital displays and the later units used the LED digital displays. The 651S-1/1A/1B is a triple-conversion superheterodyne receiver that provides continuous coverage of the 250-kHz to 29.9999-MHz frequency range. Two rotary switches provide coarse frequency adjustment of 1- and 0.1-MHz steps. The 651-S1 supports reception modes of AM, ISB (independent sideband), USB, and LSB.